The herpes viruses represent a large virus family causing widespread disease in man and in domestic and wild animals. There are more than 80 known types of herpes virus, but only eight are known to cause disease in humans. These are divided into three sub-families as shown below.
1. Alpha herpes virinaehuman herpes virus 1 (HHV-1)human simplex virus 1 (HSV-1)human herpes virus 2 (HHV-2)human simplex virus 2 (HSV-2)human herpes virus 3 (HHV-3)Varicella-Zoster virus (VZV)2. Beta herpes virinaehuman herpes virus 5 (HHV-5)cytomegalovirus (CMV)human herpes virus 6 (HHV-6)roseolovirushuman herpes virus 7 (HHV-7)3. Gamma herpes virinaehuman herpes virus 4 (HHV-4)lymphocryptovirushuman herpes virus 8 (HHV-8)rhadinovirus
Herpes virus infections are widespread and cause diseases of serious economic importance. In fact, most species of mammals become infected with at least one strain of herpes virus early in life. Infection is permanent, and there are no known cures. Anti-viral agents are available as a treatment but are usually applied and/or taken after symptoms develop (when they are least useful) and do not lend themselves for use as a prophylactic due to side effects and cost. Their use is therefore usually restricted to humans.
Current antibody detection-based diagnostic methods only detect antibody well after clinical signs appear and after the patient is infective to other animals. Other methods of detection involve polymerase chain reaction amplification of viral transcripts or viral DNA, virus isolation in tissue culture and/or virus neutralization assays.
All herpes virus infections are permanent. After a primary infection the virus enters a period of latency. Disease can be reactivated when the patient is immunocompromised or stressed or in advanced athletic training programs. Reactivation of the virus can result in recurrence of symptoms, or general malaise, or decreased exercise tolerance or performance, or it may be asymptomatic.
Determination of the presence of viral antibodies, antigens, or transcripts does not necessarily correlate with the presence of disease or clinical signs.
There are few effective vaccines available.
Drugs are available for treatment but they are costly and for maximum efficacy they need to be given prior to the appearance of major clinical signs.
Epidemiology and Clinical Manifestations
Herpes viruses cause several clinical manifestations in both normal and immunocompromised hosts. Most episodes of herpes virus infections are asymptomatic and most instances in which herpes is transmitted are from people who are unaware, or remain undiagnosed at the time of an outbreak.
Fifty to ninety percent of adult humans possess antibodies to HHV-1 or human simplex virus type 1 (HSV-1); 20%-30% of adults possess antibodies to HHV-2 or human simplex virus type 2 (HSV-2). Prevalence is greater in lower socio-economic groups and in sexually promiscuous individuals. HSV-1 is usually associated with primary infections of the orofacial area and latent infection of the trigeminal ganglion, while HSV-2 is usually associated with genital infections and latent infection in sacral ganglia. Although both primary and recurrent infections are usually self-limited, HSV can cause serious diseases such as neonatal disseminated herpes, viral encephalitis, and blinding keratitis.
During acute primary infection, HSV becomes permanently latent in the nerve root ganglia that correspond to the cutaneous or mucous membrane site of inoculation. In orolabial infection, HSV develops latency in the trigeminal ganglia, whereas latency develops in sacral ganglia after genital or anorectal infection. A variety of stimuli, such as ultraviolet light and trauma to the sensory nerve, may reactivate latent HSV. Recurrent lesions occur at or close to the primary site of infection. Recurrence seems to be related to factors which in some way decrease an individual's disease resistance, such as colds or upper respiratory infections, high levels of physical exercise, sun exposure, stress, menses in women, and in some individuals, trigger foods, particularly large quantities of chocolate or peanuts.
During reactivation, HSV replication occurs within the ganglia, and progeny virions travel peripherally along sensory nerves to the mucosal or epithelial surface innervated by the reactivated ganglion. Active virus replication at the cutaneous surface then produces the clinical signs and lesions typical of recurrent HSV infection (just as is the case with recurrent cold sores in humans infected with HSV).
“Epstein-Barr virus” (EBV) or HHV-4 is associated with infectious mononucleosis, also known as “glandular fever,” as well with oncogenesis (e.g., in Burkitt's lymphoma and nasopharyngeocarcinoma). Additionally, EBV is found in immune-suppressed patients and in patients suffering from Hodgkin's disease. EBV occurs worldwide, and most people become infected with EBV sometime during their lives. In the United States, as many as 95% of adults have been infected by the time they are 35-40 years of age. Infants become susceptible to EBV as soon as maternal antibody protection (present at birth) disappears.
EBV symptoms for infectious mononucleosis include fever, sore throat, and swollen lymph glands. Sometimes, a swollen spleen or liver involvement may develop. Heart problems or involvement of the central nervous system occurs only rarely, and infectious mononucleosis is almost never fatal. Most individuals exposed to people with infectious mononucleosis have previously been infected with EBV and are not at risk for infectious mononucleosis. In addition, transmission of EBV requires intimate contact with the saliva (found in the mouth) of an infected person. Transmission of this virus through the air or blood does not normally occur. The incubation period, or the time from infection to appearance of symptoms, ranges from 4 to 6 weeks. Persons with infectious mononucleosis may be able to spread the infection to others for a period of weeks. However, no special precautions or isolation procedures are recommended, since the virus is also found frequently in the saliva of healthy people. In fact, many healthy people can carry and spread the virus intermittently for life. These people are usually the primary reservoir for person-to-person transmission. For this reason, transmission of the virus is almost impossible to prevent.
Although the symptoms of infectious mononucleosis usually resolve in 1 or 2 months, Epstein-Barr virus remains dormant or latent in a few cells in the throat and blood for the rest of the person's life. Periodically, the virus can reactivate and is commonly found in the saliva of infected persons. This reactivation usually occurs without overt symptoms of illness but there may be non-specific symptoms such as malaise or poor athletic performance. EBV also establishes a lifelong dormant infection in some cells of the body's immune system. A late event in a very few carriers of this virus is the emergence of Burkitt's lymphoma and nasopharyngeal carcinoma, two rare cancers that are not normally found in the United States. EBV appears to play an important role in these malignancies, but is probably not the sole cause of disease.
Cytomegalovirus (CMV) or HHV-5 is found universally throughout all geographic locations and socio-economic groups, and infects between 50% and 85% of adults in the United States by 40 years of age. CMV causes infections in the lungs of immune-suppressed persons. In addition, both CMV and EBV are believed to be associated with chronic-fatigue syndrome, a malady that may afflict as many as in six out of every 100,000 people. Infectious CMV may be shed in the bodily fluids of any previously infected person, and thus may be found in urine, saliva, blood, tears, semen, and breast milk. The shedding of virus may take place intermittently, without any detectable signs, and without causing symptoms. Transmission of CMV occurs from person to person. CMV can be sexually transmitted and can also be transmitted via breast milk, transplanted organs, and rarely from blood transfusions. Although the virus is not highly contagious, it has been shown to spread in households and among young children. Transmission of the virus is often preventable because it is most often transmitted through infected bodily fluids that come in contact with hands and then are absorbed through the nose or mouth of a susceptible person. Hence, care should be taken when handling children and items like diapers. Simple hand washing with soap and water is effective in removing the virus from the hands.
CMV, which may have fewer symptoms than EBV, is always followed by a prolonged, unapparent infection during which the virus resides in cells without causing detectable damage or clinical illness. Severe impairment of the body's immune system by medication or disease consistently reactivates the virus from the latent or dormant state. CMV infection without symptoms is common in infants and young children; therefore, it is unjustified and unnecessary to exclude from school or an institution a child known to be infected. Similarly, hospitalized patients do not need separate or elaborate isolation precautions. Screening children and patients for CMV is of questionable value. The cost and management of such procedures are impractical. Children known to have CMV infection should not be singled out for exclusion, isolation, or special handling. Instead, staff education and effective hygiene practices are advised in caring for all children. Circumstances where CMV may be a problem is pregnancy, people who work with infants and children and those who are immunocompromised.
Varicella-Zoster virus (VZV) or HHV-3 causes chickenpox, typically in children, and is acquired by inhaling virus-containing particles, trapped in tiny droplets released into the air from the nose or throat of an infected person. The virus enters the body by infecting cells in the respiratory tract. From here, it spreads to many other parts of the body, including the skin, where it causes the characteristic rash. Each lesion (spot) progresses through a series of characteristic stages over about a week. Papules and vesicles develop into pustules, which then crust over prior to healing. A prominent feature of chickenpox is the development of several crops of spots, such that at the peak of the illness, 3-4 days after first appearance of the rash, there are lesions at all stages of development, from new vesicles through to crusts.
The ability of VZV to spread in this way means that chickenpox is very contagious. Virus excretion from the airways begins in the latter part of the incubation period and continues until all the spots have crusted over. Although the skin vesicles contain virus particles, they are not a major source of contagion. Scabs are not infectious. Time-honored interventions designed to minimize fever and discomfort (i.e., antipyretic medicines, cool baths and soothing lotions) are the mainstay of management.
Chickenpox is a clinical manifestation of primary infection with VZV. After recovery from primary infection, VZV is not eliminated from the body but rather, the virus lies dormant (latent), often for decades, in the roots of sensory nerves, in the spinal cord. When the infection is reactivated, it causes pain and a rash in the area supplied by the affected sensory nerves. Latent infection may result is an episode of shingles. This usually happens in older people, perhaps because, with advancing age, the immune system fails to keep the virus in check. The rash of shingles contains VZV particles, just like the rash of chickenpox. Shingles, therefore, carries a small risk of transmitting chickenpox to someone who has not had chickenpox before. Typically, an infant might acquire chickenpox by very close contact with a grandparent with shingles, but the risk of transmission is low, because VZV is not excreted from the throat during shingles.
Roseolovirus or HHV-6 is associated with “roseola” and “infantum” infections in children and with immunocompromised patients. For example, AIDS patients exhibit HHV-6 infection, although the significance of the HHV-6 infection is unclear. HHV-6 is susceptible to antiviral drugs. It is unclear, however, how antiviral drugs work against HHV-6 or how resistance to such drugs develops. A significant aspect of HHV-6 infection is its putative tie-in with multiple sclerosis and chronic fatigue syndrome, respectively.
Less is known about HHV-7 and HHV-8 (rhadinovirus). No clear evidence for the direct involvement of HHV-7 in any human disease has been reported. Studies indicate, however, that HHV-7 may be associated with HHV-6-related infections. In a related vein, HHV-8 infection is believed to be associated with Karposi's sarcoma.
In addition, herpes viruses are regarded as an important cause of wastage in the horse industry and a cause of serious compromise to athletic ability. Veterinarians, trainers and owners tend to manage horses with respiratory disease on experience alone because of the lack of clinical guides and laboratory procedures, and because there is little understanding of the relationship between viral and secondary bacterial infection and duration of disease. Alternative diagnosis or assessment procedures are often complex, invasive, inconvenient, expensive, time consuming, may expose an animal to risk of injury from the procedure, and often require transport of the animal to a diagnostic center.
In a study performed in Western Australia, herpes viruses could be isolated from the blood of 48% of horses with respiratory problems or poor performance. However, herpes viruses could also be isolated from blood of 54% of horses without clinical signs, leading to the conclusion that the presence of the virus in blood cells is not a determinant of disease. Virus isolation from nasal swabs is more indicative of respiratory infection but can only be isolated in 50% of clinical cases. It has been reported that up to 75% of horses in Britain carry the virus.
Equine rhinopneumonitis and equine abortion are commonly recognized diseases of horses caused by two distinct but antigenically-related viruses that are designated equine herpes virus type 4 (EHV-4) and equine herpes virus type 1 (EHV-1). EHV-1 is a cause of epidemic abortion, perinatal mortality, respiratory disease and, occasionally, neurological signs in horses. Abortion is the most dramatic and frightening outcome of EHV-1 infection and can be financially disastrous for breeders, with loss of clients and large insurance pay-outs. Respiratory illness caused by EHV-1, or the closely related EHV-4, can adversely affect racing performance.
The epidemiology of EHV-1 infection within the horse industry has been demonstrated by studies performed on studs in the Hunter Valley, Australia where it was demonstrated that foals are often infected with EHV-1 before 60 days of age. A separate study performed in the USA showed that 85% of foals had seroconverted by 6-8 months post-weaning. It is believed that foals become infected through exposure to respiratory droplets from mares or cohort foals.
EHV-1 is a DNA alphaherpes virus with a predilection for epithelial cells of the respiratory tract. The virus can be spread around the body to other organs by cells of the immune system. Because the viruses are related antigenically it has not been possible to date by serological examination (blood test), to determine whether a horse has been infected with either or both EHV-4 or EHV-1. For example, if a horse had been infected with EHV-4 as a foal it would develop antibodies in its serum that would react not only to EHV-4 but EHV-1 as well, so one would not know that such a foal had been infected with only EHV-4. EHV-4 has only been demonstrated to cause respiratory illness, whereas EHV-1 also causes neurological and reproductive disorders (Wilcox and Raidal, 2000, “Role of viruses in respiratory disease” RIRDC Publication No 00/146, RIRDC Project No UMU-22A; Dunowska et al., 2002, New Zealand Veterinary Journal 50 (4):132-139; Dunowska et al., 2002, New Zealand Veterinary Journal 50 (4):140-147).
EHV-1 has been shown to have a persistent, lifelong latent, infection where reactivation causes further spells of respiratory disease in the horse. However, a far more serious consequence for other horses infected by contact with the first horse (index case) occurs on breeding farms when a pregnant mare in a paddock reactivates the virus and transmits it to other in-contact pregnant mares. The index case mare may herself abort or cause abortion in one or more in contact mares. An aborted fetus and the fetal membranes and fluids are heavily infected with EHV-1 and contaminate the site where abortion occurs. Other mares in the paddock, being naturally curious, come to the site of abortion and sniff the fetus and membranes. In this way, often close to 100% of the mares in the paddock become infected and abort within 10 or 20 days causing what is commonly known as an “abortion storm”. Such outbreaks of EHV-1 abortion are of considerable economic importance to the equine, particularly Thoroughbred and Standardbred, industries worldwide.
Immunity and Diagnosis
Herpes viruses induce a strong humoral antibody response, although the effectiveness of this response in protecting the host is questionable. Protection ultimately is afforded through cell-mediated mechanisms, such as cytotoxic lymphocytes and natural killer cells. One of the key features of herpes virus infection is life-long persistent infection and latency. The virus remains in the cell nucleus and can be isolated from many organs long after clinical signs have abated. Thus, where a patient presents with non-descript clinical signs, such as poor performance or malaise, virus can be isolated from tissues but it is not clear that activation of the virus is the cause of the symptoms. Stress or other factors can lead to activation of the dormant virus and concomitant clinical signs (Walker et al., 1999, Veterinary Microbiology 68:3-13).
Infection with HSV-1 or HSV-2 induces cell-mediated immunity and the production of type-common and type-specific antibodies. Although these immune mechanisms apparently do not affect the development of HSV latency or the frequency of recurrences, they may modulate the severity of clinical recurrences and reduce HSV replication once reactivation occurs.
The host immune response elicited by HSV-1 or HSV-2 infection appears to provide partial protection against subsequent infection with HSV, as resistance to autologous infection is usually observed in HSV-infected individuals. Additionally, persons with HSV infection usually have a more mild clinical illness when infected with the alternate HSV type as compared with persons with no prior HSV infection.
HSV is the most frequently detected virus in diagnostic laboratories. Diagnosis can be made by virus isolation, polymerase chain reaction (PCR) (Espy et al., 2000, J Clin Microbiol. 38 (2):795-799) and histopathology. None of these methods are useful in the control and monitoring of the disease. Other laboratory tests available for diagnosis include specially treated scrapings that are examined under the microscope, and blood tests for antibodies. Some tests are only valid in the early stages, and more than one of these tests may be required to confirm the presence of herpes. Genital herpes can be mistaken for other diseases, including syphilis. High serum antibody levels are also an indication of a recent infection. If a person does experience visible symptoms, a culture test within the first 48 hours after symptoms appear is recommended. Beyond 48 hours, there is a risk of receiving a false negative test result because symptoms may have begun to heal and there is not enough virus left on the skin to culture.
Blood tests can be used when a person has no visible symptoms but has concerns about having herpes. Blood tests do not actually detect the virus; instead, they look for antibodies (the body's immune response) but 50 to 90% of humans have positive antibodies in the blood. There are currently two blood tests available that can give accurate results for herpes. Like any blood test, these tests cannot determine whether the site of infection is oral or genital. However, since most cases of genital herpes are caused by HSV-2, a positive result for type-2 antibodies most likely indicates genital herpes. For the most accurate result; it is recommended to wait at least 12-16 weeks from the last possible exposure to herpes to allow enough time for antibodies to develop.
The clinical diagnosis of EBV and infectious mononucleosis is suggested on the basis of the symptoms of fever, sore throat, swollen lymph glands, and the age of the patient. Usually, laboratory tests are needed for confirmation. Serologic results for persons with infectious mononucleosis include an elevated white blood cell count, an increased percentage of certain atypical white blood cells, and a positive reaction to a “mono spot” test.
Clinical diagnosis of CMV is by the enzyme-linked immunosorbent assay (or ELISA), a serologic test for measuring antibodies. The result can be used to determine if acute infection, prior infection, or passively acquired maternal antibody in an infant is present. Other tests include various fluorescence assays, indirect hemagglutination, and latex agglutination.
Diagnosis of EHV is based on respiratory symptoms i.e. cough or nasal discharge. However, it is important to distinguish between respiratory bacterial or viral infections; exercise induced pulmonary hemorrhage and allergy. Cost of misdiagnosis is large. Costs to owners to diagnose the condition include transport, veterinary advice and pathology tests. Respiratory disease can kill quickly, create life-long disability, impede performance or require long periods of rest. Horses in an active carrier state can re-infect other animals.
Using the horse industry as an example, there is a need for accurate, type-specific serological surveillance of horses for the presence of EHV-4 and/or EHV-1 antibodies to assist in our understanding of the epidemiology of these viruses, particularly EHV-1. EHV-1 infections are difficult to diagnose and treat, and any useful information on how to manage horses with the disease would be welcomed by the industry. Presently, however, EHV-1 or EHV-4 antibodies in polyclonal serum cannot be differentiated because of the extensive antigenic cross-reactivity between the two viruses. The availability of such a specific serological test would also have profound implications in the control, perhaps eradication, of EHV-1 and in the selection of candidate horses for vaccination. Although there is a short lived period following infection when horses are protected against EHV-1 there is generally not a sufficiently high level of long-term immunity to consistently protect against EHV-1 disease. Horses can therefore be re-infected several times during their lifetime, and vaccination strategies are complicated by the ability of herpes viruses to establish a lifelong latent infection in the host animal.
There is no vaccine that prevents HSV disease from occurring. Although several protein subunit vaccines based on HSV-2 envelope glycoproteins have reached advanced-phase clinical trials. These antigens were chosen because they are the targets of neutralizing-antibody responses and because they elicit cellular immunity.
Oral anti-viral medications such as acyclovir, famcyclovir, or valacyclovir have been developed to effectively treat herpes infections. These medications can be used to treat an outbreak or can be used for suppressing herpes recurrences. Lower doses may be helpful in reducing the number of herpes attacks in people with frequent outbreaks. Gancyclovir, penciclovir and acyclovir are effective inhibitors of herpes simplex virus types 1 (HSV-1) and 2 (HSV-2). This antiviral therapy is expensive and needs to be given upon onset of the earliest clinical signs. These antiviral therapies are based on the use of suicide genes, such as the thymidine kinase gene. The efficacies of gancyclovir, pencyclovir and acyclovir in inducing cell death in the herpes simplex virus thymidine kinase (HSVTK) system have been compared (Shaw et al. 2001, Antivir Chem. Chemother. 12 (3):175-86). All compounds delay growth or reduced viability of HSVTK-transformed cells.
There is no specific treatment for EBV and infectious mononucleosis, other than treating the symptoms. No antiviral drugs or vaccines are available. Some physicians have prescribed a 5-day course of steroids to control the swelling of the throat and tonsils. The use of steroids has also been reported to decrease the overall length and severity of illness, but these reports have not been published. Finally, even when EBV antibody tests, such as the early antigen test, suggest that reactivated infection is present, this result does not necessarily indicate that a patient's current medical condition is caused by EBV infection. A number of healthy people with no symptoms have antibodies to the EBV early antigen for years after their initial EBV infection.
Currently, no treatment exists for CMV infection in the healthy individual. Antiviral drug therapy is now being evaluated in infants. Gancyclovir treatment is used for patients with depressed immunity and who have either sight-related or life-threatening illnesses. Vaccines are still in the research and development stage.
Chickenpox is not usually treated with a specific antiviral compound owing to its short duration and generally mild, uncomplicated nature. Some doctors believe that antiviral medication may be appropriate for older patients, in whom the disease tends to be more severe. A vaccine for chicken pox (varicella vaccine) has been available since 1995. Studies show that the varicella vaccine is 85% effective in preventing disease. The vaccine may be beneficial to non-immune adults, in particular those at high risk, for example child care and health workers. Because most adults are immune, checking serological status before vaccination is recommended.
The principal challenge in the management of shingles is rapid resolution of pain. Four factors independently increase the risk of persistent pain: advancing age, severe or moderately severe pain at the time the rash appears (called acute pain), pain before the rash appears (called prodromal pain) and failure to obtain adequate antiviral treatment within three days of appearance of the rash. Pain, particularly persistent pain, is thought to be largely the result of virus-induced damage to the affected nerve. The rationale behind the use of antiviral agents is simple: by stopping virus replication as quickly as possible, nerve damage is minimized. Shingles does respond to oral anti-viral medications, namely acyclovir, famciclovir and valacyclovir.
Early identification of EHV-1 infection, especially viral abortion, is important in managing horses so that cyclic re-infection of susceptible horses and relapse do not occur. The cost of EHV infection to the horse industry is large through lost training days, re-infection and recurrent illness, abortions and poor performance. Treatment largely depends on accurate diagnosis. Vaccines are available that elicit strong humoral immune responses, but these are not fully protective. Breakthrough infections commonly occur in vaccinated animals. Despite the availability of vaccines it is generally known that they afford little protection, and breakthrough infections commonly occur in vaccinated animals. Antibiotic treatment only prevents secondary bacterial infection.
Currently, methods for diagnosing herpes virus and associated diseases in the blood are based on antibody-antigen quantitation or detection of viral genetic information (e.g., by polymerase chain reaction). For example, U.S. Pat. No. 6,506,553 describes an assay for diagnosis of EBV and associated diseases by detecting antigen antibodies in a blood sample. The assay detects IgG and IgM antibodies to the diffuse (EA-D) and restricted (EA-R) components of the early antigen of EBV in blood, and more specifically in serum. This assay can be used for diagnosis of EBV-associated disease; such as infectious mononucleosis (IM) for example, and can also be utilized to distinguish between individuals in the acute versus the convalescent phase of disease. However this patent describes a method for the detection of early antigen antibodies, not virus or immune reaction to the virus.
U.S. Pat. No. 6,537,555 describes a composition and method for the diagnosis and treatment of HSV infection based on the detection of HSV antigens. This patent, however, does not describe a method for the detection of virus or immune reaction to the virus.
International Publication WO 99/45155 describes a method for gene expression and molecular diagnostic approaches for the amplification and detection of EBV nucleic acid, in particular RNA-specific sequences. This method is specifically suited for the detection of late stage infection of EBV gene expression in circulating peripheral blood cells, in human (tumour) tissue samples and thin sections thereof using “in solution” amplification or “in situ” amplification techniques and in other biological samples potentially containing EBV-infected cells. However, this method only detects viral transcripts and is most suitable for late stage disease diagnosis. It also does not detect the immune reaction to viral infection, which causes the earliest symptoms of malaise, fever and swollen lymph nodes.
U.S. Patent Application Publication 20040072147 discloses the use of a probe oligonucleotide and at least two primer oligonucleotides for selectively directing the amplification of the target segment of a particular herpes virus type or strain including: HSV-1, drug resistant HSV-1, HSV-2, drug resistant HSV-2, VZV, EBV (HHV-4a and HHV-4b), CMV, lymphocryptovirus (HHV-6a, HHV-6b), HHV-7, and rhadinovirus (HHV-8). However, this method does not provide any insight into the stage of disease or when the animal was infected, or whether the disease is active.
U.S. Pat. No. 6,193,983 describes a method for detecting EHV-4 and EHV-1 type-specific glycoproteins for clinical applications associated with the characterization of such glycoproteins. This method detects specific antibodies to EHV-1 and EHV-4 but it is well known that most horses are exposed to these viruses at an early age and antibody titres persist for some time. Thus, this method provides no insight into the stage of disease or when the animal was infected, or whether the disease is active.
In summary, infection with various strains of herpes virus is a widespread phenomenon in the community and causes disease of serious economic importance. Most humans and domestic animals become infected with at least one strain of herpes virus at an early age and the infection is life-long. Herpes viruses enter a stage of latency and can be reactivated causing a recurrence of symptoms. Often reactivation of the virus can be asymptomatic, but can manifest as malaise or as non-specific symptoms such as low grade fever, lethargy, chronic fatigue, poor exercise tolerance, or poor athletic performance. Physiological stress (such as heavy exercise, concurrent disease, mental stress), or where the immune system is compromised (for example HIV infection, immunosuppressive therapy) can lead to reactivation of herpes viruses leading to chronic symptoms of infection. For these reasons, it is often important to monitor herpes virus infections, especially in immunocompromised patients or elite athletes. Current antibody-based diagnostic methods do not lend themselves to monitoring herpes virus infections because they measure serum antibodies that become elevated 7-14 days following infection and remain elevated due to viral latency. Other current diagnostic methods, such as virus isolation and PCR, do not lend themselves to monitoring as they are laborious or the viral genome is not consistently present in blood cells. Results derived from current diagnostic methods do not correlate with the timing of onset of clinical signs. For example, antibodies can first be detected 10-14 days following initial infection and persist for long periods. A single antibody measurement does not indicate when infection occurred or the level of disease activity and measuring viral transcripts or viral proteins also does not indicate the level of disease activity. The immune system of the host is ultimately responsible for protection from viral invasion. It is the immune response, rather than the virus itself, that is responsible for clinical signs of disease. A more appropriate monitoring tool for herpes virus infection would be one that measured specific host immune reactions to infection.
As such, there currently exists a need for more effective modalities for diagnosing herpes virus infections, for determining active herpes virus infection through host immune response, and for identifying animals amenable to treatment or prophylactic therapy with antiviral agents. Primary infection leads to latent infection in all cases and, as such, there is often the risk of relapse in immunocompromised patients. In such cases, symptoms may or may not be evident, detectable or communicable. Accordingly, there is currently a need for better processes and reagents for assessing and monitoring mammals at risk of herpes virus infection and/or relapse.